The ironing process is used in the industry to reduce the thickness of deep drawn products and increase their height in such products as beverage cans, precision tubes and so forth. It could also be used in manufacturing sheet and bulk products with precise dimensions and smooth surfaces like in hole flanging and gear surface smoothing).
Thin high-walled components are difficult to be produced by deep drawing. For example any occasional inhomogeneity in the material properties and/or in its geometry promotes premature rupture instability. So, the deep drawing operation could be performed more safely on a thicker blank and with a modest drawing ratio, and later preformed cups would be significantly deepened by ironing.
In this way, stronger cups with more dimensional accuracy and surface smoothness would be produced. The extent to which products are deepened in each stage of ironing process depends on the amount of reduction in thickness achievable in that particular stage. Thus, when considerable rise in height is required, there will be a need for a high reduction in thickness which will call for a variety of dies and raise the manufacturing expenses.
In conventional ironing processes only a fraction of cup wall thickness is reduced. As a result, in order to reach to high reductions in thickness, several ironing stages are needed and the workpiece must go through several cleaning, heat treatment and lubrication stages.
Many efforts were done over the years to improve the ironing process. Some of them evaluated process parameters through numerical and experimental investigations to reach optimum final results. For example, a computer-aided model of ironing offered by Kampus was used to investigate the effects of process parameters on inner and outer diameter of workpiece.
Huang also used a finite-element method to study ironing and offered some considerations for die design. In another effort, Van der Aa presented graphs for the ironing process of polymer coated sheet metals. Another effort was due to Folle in which he studied the manufacturing process of beverage cans. He studied the influence of parameters such as the angle of the ironing die, friction coefficient and the clearance between the punch and ironing die on the ironing force.
Using the conventional ironing process, only a fraction (30% for aluminum and 36% for steel) of cup wall thickness is reduced. Therefore, there were several suggestions in previous works to overcome the problem of low reduction ratio in the ironing process. Using several rings arranged one after the other would be the first action that raises the thickness reduction by approximately 12%. Another way offered by Kampu{hacek over (s)} et al. is by using an imposed force on the cup edge for better leading of the material to forming zone.
Besides, increasing the friction between the punch and the product is another frequently suggested way for enhancing the reduction in thickness. This increased friction will transfer more traction from the punch to the product by interfacial shear. Based on this suggestion, the punch load acting on the bottom of the cup (at a given reduction) is reduced, thereby allowing higher reductions before the onset of rupture in the ironed wall.
One of the efficient ways of reaching such an increase in friction would be by applying a fluid pressure on the specimen. The latter way is frequently used in sheet forming processes, notably in deep drawing, to increase the process efficiency. Nonetheless, supplying the high pressures during the process is a great concern, technically and economically. So, it is worthwhile to go after new hydro mechanical tooling to overcome the expenses of high pressure equipment.
Studies on using fluid pressure in the ironing process are not as vast as that of deep drawing process. Tirosh et al offered a method in which a fluid pressure surrounded the cup on the mandrel. This pressure theoretically increased the frictional shear on cup-mandrel interface and also pushed the edge of the cup to forming zone. This resulted in an increase in the thickness reduction in the ironing process.
But the problem is that in practice, the fluid will be in contact with both sides of the specimen. So, the cup-punch interface will become lubricated during the process which is a basic drawback in the ironing process. Moreover, the fluid pressure will act on both sides of the specimen wall and as a result it cannot have a significant effect on increasing the friction on cup-punch interface.
Therefore, only the backward fluid pressure on the specimen edge would be helpful for increasing workability of the material. Thus, huge pressures (6000 bar for 60% reduction in steel specimens) are needed in order to significantly increase the workability of the material.